Septic tanks have long been utilized as the primary means for residential sewage disposal in areas to which sewer lines have not been extended. A normal septic tank system employs only a single tank and the liquid effluent from such tank is drained through a leaching field. The primary function of the basic septic tank is to subject the solid materials in the sewage to biodegradation action of bacteria which normally develops in such tanks or which can be applied thereto. The bacterialogical action normally converts the great majority of solid materials into relatively harmless soluble materials, gases and insoluble material. The insolubles remaining after the completion of the bacterial action collect in the bottom of the tank as a sludge which must be periodically removed.
The effectiveness of most septic tanks, however, is not limited by the accumulation of sludge but rather by the accumulation upon the top of the effluent of a scum or skin which comprises oily or fatty liquids and particles of untreated or partially-treated solids surrounded by gases resulting in a flotation of such particles to the surface of the effluent. The existence of such scum or skin is highly deleterious to the effective action of the bacteriological degradation process in the septic tank because the bacteriological action is greatly diminished to the point of suffocation. The existence of a heavy scum or skin layer on the top of the effluent effectively prevents the generation of gases, which are primarily methane and carbon dioxide, by anaerobic bacteria mainly in the sludge. The generated gases cannot escape and the high gas concentration adversely effects the bacteriological action. The result is that the tank scoures, not because of the accumulation of sludge, but because of the concentration of scum or skin on the surface of the tank fluid, prevents effective bacteriological degradation. In severe situations, the discharge of the tank may have such a high concentration of untreated sewage as to cause spoilage of the leaching bed.
The adverse effects of scum on the surface of a sewage effluent has been well recognized in commercial and municipal sewage treatment systems to the extent that elaborate mechanical arrangements have been provided to move scrapers or buckets across the top of the effluent to remove the scum therefrom and discharge it exteriorly of the treatment tank or basin and separate from the normal effluent. Unfortunately, any type of mechanical system for removal of scum is far too expensive to be practical for typical home sewage systems; more importantly, the requirement for moving components necessarily means that constant maintenance would be required to keep the system in operable condition.
Typical mechanical systems of the type heretofor discussed may be found in U.S. Pat. No. 2,237,172 to Briggs, U.S. Pat. No. 2,337,859 to Stuller, U.S. Pat. No. 2,455,338 to Lind and U.S. Pat. No. 2,780,361 to Evans et al.
In Briggs U.S. Pat. No. 2,237,172, a sewage settling tank is provided with motor driven flights which skims scum from sewage to the entry to an ascending chute across which a second set of motor driven flights extend to lift the scum over the lip of an open trough above the liquid level within the tank. The liquid is withdrawn through an effluent port spaced laterally from the trough, and at a level below the trough.
Stuller U.S. Pat. No. 2,337,859 discloses a tank with flights extending through the liquid level surface defined by a weir in an outlet channel and motor driven toward an auxiliary scum remover pipe spaced from the outlet channel. The scum remover has elongate wide-mouthed apertures in the pipewall which extend longitudinally of the pipe above the liquid surface level in one condition and can be lowered to and below that surface by rotation of the pipe around its longitudinal axis.
Lind U.S. Pat. No. 2,455,338 is similar to Stuller in general arrangement but has a float actuated scum remover pipe for driving the pipe in rotation by means of a limit switch, electric motor and gearing rather than the manual rotating means of Stuller.
Evans et al. U.S. Pat. No. 2,780,361 has a scum remover separate from the outlet port for effluent from a waste liquid treatment tank. A motor driven skimmer board is arranged to rotate about the central axis of a circular tank to carry the scum on the liquid surface to a scum-receiving pipe having a ported area parallel to its longitudinal axis. The port may be in the form of a continuous wide mouthed slot and the pipe is arranged with adjusting means to raise or lower the port with respect to the liquid surface in the tank.
In addition to the elongate scum removers extending laterally over a substantial portion of treatment tanks, there have been removers of limited extent as shown in U.S. Pat. No. 1,188,159 to Dittrick, U.S. Pat. No. 2,439,633 to Reinhart, U.S. Pat. No. 2,717,873 to Montgomery et al., and U.S. Pat. No. 3,433,258 to Steel.
Dittrick U.S. Pat. No. 1,188,159 and Reinhart U.S. Pat. No. 2,717,873 each shows a self cleaning sanitary cistern including an exit at a lower portion and a funnel at the liquid surface to drain material from the upper surface as a skimmer. The funnel is of limited lateral extent and would not collect scum across a broad area of the liquid surface.
Montgomery et al. U.S. Pat. No. 2,717,873 discloses a clarifier and digester having an open ended, and longitudinally slotted, short pipe extending into a tank at a region remote from the primary exit of liquid.
Steele U.S. Pat. No. 3,433,258 discloses a tank which may be used as a septic tank and is provided with a stub pipe outlet having an array of circumferentially spaced longitudinal slots. The pipe has a closed end containing a central aperture and the slot array extends around the periphery to form an outlet strainer rather than a scum remover over a broad expanse of the liquid surface in the tank.
While multistage sewage treatment arrangements are known, particularly for community and industrial systems, these systems generally include mechanisms having the mechanical pumps, scrapers, and scum removal systems of the type discussed above. Domestic septic systems are known with multi compartmented septic tanks and even multi tanks connected in series. However, the inhibiting of anaerobic digestion by the development of scum which inhibits the escape of gas from the liquid has been found to progress in such serially related tanks such that each tank develops a scum at a progressively lower rate if direct flow from its preceding tank is permitted. Ultimately, the scum is passed to all tanks with the anerobic action deteriorating in the tank first to receive the waste fluids to a point that it becomes essentially a solids trap and the intended initial breakdown of the sewage is transferred to the next tank of the series. When permitted to continue, ultimately all tanks have their anaerobic activity blocked. German Pat. No. 1,036,771 of Aug. 14, 1958 shows a multicompartmented septic tank of this general construction wherein a thick layer of scum has developed on the surfaces of the first compartment and, although the sewage passing to the second compartment is drawn from the lower portion of the first compartment it too develops a scum layer. The German disclosure appears to contemplate a high density scum since it would appear that the gravity overflow exit to the following compartment of the series is located at the scum surface.
In accordance with the above, an object of this invention is to improve scum removal over the upper surface of the liquid in a septic tank system.
A second object of this invention is to minimize the development of scum in the initial compartment or tank of a septic system to maintain anerobic action in that tank by passing metered quantities of scum to a next compartment for breakdown. This removal of scum from the preceding section mitigates against the inhibiting buildup in the first section while maintaining levels in the succeeding section which can be broken down by anaerobic action.
Another object is to eliminate mechanical movement of the scum removal equipment and the attendant complexity and maintenance burdens of such equipment.
A further object is to enhance the anaerobic digestion of sewage by removal of scum and the avoidance of the buildup of suffocating scum layers in septic tanks.